The primary objective of this paper is to suggest a mechanism by which current trends in public education, in particular the rise of externally defined numerical metrics of achievement to define school success, operate to shape school policy, curriculum and practice. Here, we argue that the modern school's focus on the cognitive dimension of learning in combination with the extensive use of standardized tests as primary evidence of a quality education exploit students as a particular kind of commodity: one that is held in common. This opens the door for a pernicious set of predictable outcomes, known collectively as "tragedies of the commons", first described by the ecologist Garrett Hardin in 1968. From our perspective as researchers, we first describe how a tragedy of the commons can arise in schools. Next, we present a description of a tragedy of the commons at a particular school, illustrating how the school deals with external accountability pressures as would be predicted by the logic of the tragedy of the commons, and how these pressures shape school policy, curriculum and practice. Finally, we suggest how these pressures might be attenuated by purposefully building a strong sense of school community that fosters communication and trust.